In these apparatuses, it is necessary to take off a purge stream permanently from the external evaporators used to evaporate the liquid oxygen at low pressure and/or from evaporators within the columns, in order to avoid a potentially dangerous accumulation of impurities. These purge streams are withdrawn substantially permanently, for example, from the bottom of a low-pressure column, which produces only gaseous oxygen withdrawn directly from that column, in order to prevent the accumulation of impurities.
The purge of the cryogenic liquids from the cold box are generally sent to a specific device for evaporating them:                gravel pit, for generators of very small size        atmospheric chamber, which collects the liquids, which will subsequently evaporate slowly        evaporator (using heating and ventilation).        
In EP-A-0605262, a purge liquid is sent into a storage facility, and a liquid portion from the storage facility is sent into a heat exchanger to be mixed with gaseous oxygen. The storage facility is also fed by liquid transported by a truck; the system is therefore one of injection of liquid oxygen with an extra contribution of external liquid, thus allowing the purge to be evaporated with the reheated gaseous oxygen.
In EP-A-1202012, a storage facility is fed by a purge stream originating from the bottom of the low-pressure column in the case of a reduction in the production of the column system, a standby stream which is evaporated in a standby evaporator is withdrawn from the storage facility.